Several in vitro systems were explored for their applicibility to the testing of (suspected) neurotoxic substances, such as Erythrosin B (FD and D Red 3) (EB) an artificial halogenated fluorescein derivative. The dye is also useful for solublizing and partially purifying ATPase from rat brain cortex. It also inhibits ATPase and calcium transport activity of rabbit muscle sarcoplasmic reticulum. In vitro toxic effects are also monitored by studying the growth of chick dorsal root explants. Storage and release of catecholamines from adrenal medullary cells are affected by a variety of heavy metals, partially through interference with calcium-specific mechanisms involved in release of the neurotransmitter. The calcium-promoted fusion of isolated chromaffin granules, and its inhibition by various heavy metals, is being studied as a model process for exocytotic release of catecholamines in vivo. The kinetics of calcium-promoted fusion have been studied using fluorescence energy transfer techniques.